"Omen" before Shma

Is it ancient or relatively new custom to say "Omen" that is "Ke-l - melekh neeman" in the end of the second brakhah before Shma? I know it's forbidden to say "Omen" after Shma before the third brakhah or in the end of Kriyat Shma before Amidah not to make any interruption. Can you explain the meaning of the "Omen"="Ke-l melekh neeman"?

Recently I've found in Siddur Minhog Sfarad that Shma consists from 245 words and to make 248 words someone adds three words "kE-l melekh neeman" because of Kabboloh.

The poskei Ashkenaz such as the Rosh and Tur held that one should answer omen to Ga'al Yisroel, and that it is not considered a hefsek.